Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/707

Rh But, further, there is very strong reason for believing that the influence of temperature on life is greatly modified, first, by the nature of the medium in which organisms are placed, and, secondly, by the length of time during which any given temperature is kept up. On this point recent experiments made by Dr Roberts of Manchester are of great importance. He found, for example, as every other careful experimenter has done, that ordinary infusion of hay boiled for a few minutes was sterilized, that is to say, no development of Bacteria took place in it, however long it might be kept ; while if the infusion was rendered alkaline with ammonia or liquor potassae, it was not sterilized except after an exposure to the heat of boiling water for more than an hour. Some times it became productive after two hours, and once after three hours of such exposure. Is it to be imagined that, in the case of the alkalized hay infusion, the heat applied really killed the Bacteria which existed in the infusion, and that Bacteria of identically the same kind were generated afresh out of the dead matter 1 ? or is it more probable that the powers of resistance of the Bacteria to heat were simply increased by the alkalinity of the infusion] The statement of the questions surely render it unnecessary to answer them. Dr Roberts further proves that there are two factors in the induction of sterilization, the degree of heat on the one hand, and the duration of its application on the other. A longer exposure to a lower temperature was equivalent to a shorter exposure to a higher temperature. &quot;For example, speaking roughly, an exposure of an hour and a half to a heat of 212 Fahr. appeared to be equivalent to an ex posure for fifteen minutes to a heat of 228 Fahr. It is hard to conceive what explanation can be offered of this fact, except that, under the conditions of the experiment, the organisms were either all affected by the first incidence of the heat in such a way as only to arrest some of their vital functions, and to leave a potentiality of life in them, such as exists in some kinds of dried living matter ; or that they individually differed very much in their powers of resistance, and that some were able to withstand heat much longer than others. Under these circumstances it will be evident, that no experimental evidence that a liquid may be heated to n degrees, and yet subsequently give rise to living organisms, is of the smallest value as proof that abiogenesis has taken place, and for two reasons : Firstly, there is no proof that organisms of the kind in question are dead, except their permanent incapacity to grow and reproduce their kind ; and secondly, since we know that conditions may largely modify the power of resistance of such organisms to heat, it is far more probable that such conditions existed in the experiment in question, than that the organisms were generated afresh out of dead matter. Not only is the kind of evidence adduced in favour of abiogenesis logically insufficient to furnish proof of its occurrence, but it may be stated as a well-based induction, that the more careful the investigator, and the more com plete his mastery over the endless practical difficulties which surround experimentation on this subject, the more certain are his experiments to give a negative result ; while positive results are no less sure to crown the efforts of the clumsy and the careless. It is argued that a belief in abiogenesis is a necessary and corollary from the doctrine of Evolution. This may be true of the occurrence of abiogenesis at some time ; but if the present day, or any recorded epoch of geological time, be in question, the exact contrary holds good. If all living beings have been evolved from pre-existing forms of life, it is enough that a single particle of living protoplasm should once have appeared on the globe, as the result of no matter what agency. In the eyes of a consistent evolutionist any further independent formation of protoplasm would be sheer waste.

The production of living matter since the time of its first appearance, only by way of biogenesis, implies that the specific forms of the lower kinds of life have undergone but little change in the course of geological time, and this is said to be inconsistent with the doctrine of evolu tion. But, in the first place, the fact is not inconsis tent with the doctrine of evolution properly understood, that doctrine being perfectly consistent with either the progression, the retrogression, or the stationary condition of any particular species for indefinite periods of time; and secondly, if it were, it would be so much the worse for the doctrine of evolution, inasmuch as it is unquestionably true, that certain, even highly organized, forms of life have persisted without any sensible change for very long periods. The Terebratula psittacea of the present day, for example, is not distinguishable from that of the Cretaceous epoch, while the highly organised Teleostean fish, Beryx, of the Chalk differed only in minute specific characters from that which now lives. Is it seri ously suggested that the existing Tcrebratulce and Beryccs are not the lineal descendants of their Cretaceous ancestors, but that their modern representatives have been inde pendently developed from primordial germs in the interval ? But if this is too fantastic a suggestion for grave con sideration, why are we to believe that the Gloligerince of the present day are not lineally descended from the Cretaceous forms ? And if their unchanged generations have succeeded one another for all the enormous time represented by the deposition of the Chalk and that of the Tertiary and Quaternary deposits, what difficulty is there in supposing that they may not have persisted unchanged for a greatly longer period? The fact is, that at the present moment there is not a shadow of trustworthy direct evidence that abiogenesis does take place, or has taken place, within the period during which the existence of life on the globe is recorded. But it need hardly be pointed out, that the fact does not in the slightest degree interfere with any conclusion that may be arrived at deductively from other considerations that, at some time or other, abiogenesis must have taken place. If the hypothesis of evolution is true, living matter must have arisen from not-living matter ; for by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it, life being entirely incompatible with the gaseous state. But living matter once originated, there is no necessity for another origina tion, since the hypothesis postulates the unlimited, though perhaps not indefinite, modifiability of such matter.

Of the causes which have led to the origination of living matter, then, it may be said that we know absolutely of nothing. But postulating the existence of living matter endowed with that power of hereditary transmission, and with that tendency to vary which is found in all such matter, Mr Darwin has shown good reasons for believing that the interaction between living matter and surrounding conditions, which results in the survival of the fittest, is sufficient to account for the gradual evolution or plants and animals from their simplest to their most complicated forms, and for the known phenomena of Morphology, Physiology, and Distribution. 